The dominant contribution of Southern Ocean heat uptake to time‐evolving radiative feedback in CESM

Ocean uptake of anthropogenic heat over the past 15 years has mostly occurred in the Southern Ocean, based on Argo float observations. This agrees with historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), where the Southern Ocean (south of 30°S) accounts for 72% ± 28% of global heat uptake, while the contribution from the North Atlantic north of 30°N is only 6%. Aerosols preferentially cool the Northern Hemisphere, and the effect on surface heat flux over the subpolar North Atlantic opposes the greenhouse gas (GHG) effect in nearly equal magnitude. This heat uptake compensation is associated with weakening (strengthening) of the Atlantic meridional overturning circulation (AMOC) in response to GHG (aerosol) radiative forcing. Aerosols are projected to decline in the near future, reinforcing the greenhouse effect on the North Atlantic heat uptake. As a result, the Southern Ocean, which will continue to take up anthropogenic heat largely through the mean upwelling of water from depth, will be joined by increased relative contribution from the North Atlantic because of substantial AMOC slowdown in the twenty-first century. In the RCP8.5 scenario, the percentage contribution to global uptake is projected to decrease to 48% ± 8% in the Southern Ocean and increase to 26% ± 6% in the northern North Atlantic. Despite the large uncertainty in the magnitude of projected aerosol forcing, our results suggest that anthropogenic aerosols, given their geographic distributions and temporal trajectories, strongly influence the high-latitude ocean heat uptake and interhemispheric asymmetry through AMOC change.

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Dependence of regional ocean heat uptake on anthropogenic warming scenarios

Science Advances ◽

10.1126/sciadv.abc0303 ◽

2020 ◽

Vol 6 (45) ◽

pp. eabc0303

Author(s):

Xiaofan Ma ◽

Wei Liu ◽

Robert J. Allen ◽

Gang Huang ◽

Xichen Li

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Meridional Overturning Circulation ◽

Anthropogenic Aerosols ◽

Ocean Heat Uptake ◽

The North ◽

Heat Uptake ◽

The North Atlantic ◽

Aerosol Effects ◽

Business As Usual

The North Atlantic and Southern Ocean exhibit enhanced ocean heat uptake (OHU) during recent decades while their future OHU changes are subject to great uncertainty. Here, we show that regional OHU patterns in these two basins are highly dependent on the trajectories of aerosols and greenhouse gases (GHGs) in future scenarios. During the 21st century, North Atlantic and Southern Ocean OHU exhibit similarly positive trends under a business-as-usual scenario but respectively positive and negative trends under a mitigation scenario. The opposite centurial OHU trends in the Southern Ocean can be attributed partially to distinct GHG trajectories under the two scenarios while the common positive centurial OHU trends in the North Atlantic are mainly due to aerosol effects. Under both scenarios, projected decline of anthropogenic aerosols potentially induces a weakening of the Atlantic Meridional Overturning Circulation and a divergence of meridional oceanic heat transport, which leads to enhanced OHU in the subpolar North Atlantic.

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Connecting tropical climate change with Southern Ocean heat uptake

Geophysical Research Letters ◽

10.1002/2017gl074972 ◽

2017 ◽

Vol 44 (18) ◽

pp. 9449-9457 ◽

Cited By ~ 14

Author(s):

Yen-Ting Hwang ◽

Shang-Ping Xie ◽

Clara Deser ◽

Sarah M. Kang

Keyword(s):

Climate Change ◽

Southern Ocean ◽

Tropical Climate ◽

Ocean Heat Uptake ◽

Heat Uptake ◽

Tropical Climate Change

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Recent trajectory of ocean heat uptake estimated from novel 1972-2017 ocean sea-ice model hindcast simulations

10.5194/egusphere-egu21-8096 ◽

2021 ◽

Author(s):

Maurice Huguenin ◽

Ryan Holmes ◽

Matthew England

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Heat Content ◽

Anthropogenic Heat ◽

Global Ocean ◽

Ocean Heat Uptake ◽

Heat Uptake ◽

The Tropics ◽

And Storage ◽

Ice Model

<p>Uptake and storage of heat by the ocean plays a critical role in modulating the Earth's climate system. In the last 50 years, the ocean has absorbed over 90% of the additional energy accumulating in the Earth system due to radiative imbalance. However, our knowledge about ocean heat uptake (OHU), transport and storage is strongly constrained by the sparse observational record with large uncertainties. In this study, we conduct a suite of historical 1972&#8211;2017 hindcast simulations using a global ocean-sea ice model that are specifically designed to account for a cold start climate and model drift. The hindcast simulations are initialised from an equilibrated control simulation that uses repeat decade forcing over the period 1962-1971. This repeat decade forcing approach is a compromise between an early unobserved period (where our confidence in the forcing is low) and later periods (which would result in a shorter experiment period and a smaller fraction of the total OHU). The simulations are aimed at giving a good estimate of the trajectory of OHU in the tropics, the extratropics and individual ocean basins in recent decades. Many modelling studies that look at recent OHU rates so far use a simpler approach for the forcing. For example, they use repeating cycles of 1950-2010 Coordinated Ocean Reference Experiment (CORE) forcing that is consistent with the Ocean Model Intercomparison Project 2 (OMIP-2). However, this approach cannot account for model drift. The new simulations here highlight the dominant role of the extratropics, and in particular the Southern Ocean in OHU. In contrast, little heat is absorbed in the tropics and simulations forced with only tropical trends in atmospheric forcing show only weak global ocean heat content trends. Almost 50% of the heat taken up from the atmosphere in the Southern Ocean is transported into the Atlantic Ocean. Two-thirds of this Southern Ocean-sourced heat is then subsequently lost to the atmosphere in the North Atlantic but nevertheless this basin gains heat overall. Our results help to estimate the large-scale cycling of anthropogenic heat within the ocean today and have implications for heat content trends under a changing climate.</p>

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The effects of historical ozone changes on Southern Ocean heat uptake and storage

Climate Dynamics ◽

10.1007/s00382-021-05803-y ◽

2021 ◽

Author(s):

Shouwei Li ◽

Wei Liu ◽

Kewei Lyu ◽

Xuebin Zhang

Keyword(s):

Southern Ocean ◽

Ocean Heat Uptake ◽

Heat Uptake ◽

And Storage

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The role of vertical eddy flux in Southern Ocean heat uptake

Geophysical Research Letters ◽

10.1002/2013gl057706 ◽

2013 ◽

Vol 40 (20) ◽

pp. 5445-5450 ◽

Cited By ~ 31

Author(s):

A. K. Morrison ◽

O. A. Saenko ◽

A. McC. Hogg ◽

P. Spence

Keyword(s):

Southern Ocean ◽

Eddy Flux ◽

Ocean Heat Uptake ◽

Heat Uptake ◽

Vertical Eddy

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A Conceptual Model of Ocean Heat Uptake under Climate Change

Journal of Climate ◽

10.1175/jcli-d-13-00344.1 ◽

2014 ◽

Vol 27 (22) ◽

pp. 8444-8465 ◽

Cited By ~ 39

Author(s):

David P. Marshall ◽

Laure Zanna

Keyword(s):

Southern Ocean ◽

Wind Forcing ◽

System Model ◽

Ekman Transport ◽

Earth System ◽

Diapycnal Mixing ◽

Ocean Heat Uptake ◽

Ocean Stratification ◽

Heat Uptake ◽

Ocean Wind

Abstract A conceptual model of ocean heat uptake is developed as a multilayer generalization of Gnanadesikan. The roles of Southern Ocean Ekman and eddy transports, North Atlantic Deep Water (NADW) formation, and diapycnal mixing in controlling ocean stratification and transient heat uptake are investigated under climate change scenarios, including imposed surface warming, increased Southern Ocean wind forcing, with or without eddy compensation, and weakened meridional overturning circulation (MOC) induced by reduced NADW formation. With realistic profiles of diapycnal mixing, ocean heat uptake is dominated by Southern Ocean Ekman transport and its long-term adjustment controlled by the Southern Ocean eddy transport. The time scale of adjustment setting the rate of ocean heat uptake increases with depth. For scenarios with increased Southern Ocean wind forcing or weakened MOC, deepened stratification results in enhanced ocean heat uptake. In each of these experiments, the role of diapycnal mixing in setting ocean stratification and heat uptake is secondary. Conversely, in experiments with enhanced diapycnal mixing as employed in “upwelling diffusion” slab models, the contributions of diapycnal mixing and Southern Ocean Ekman transport to the net heat uptake are comparable, but the stratification extends unrealistically to the sea floor. The simple model is applied to interpret the output of an Earth system model, the Second Generation Canadian Earth System Model (CanESM2), in which the atmospheric CO2 concentration is increased by 1% yr−1 until quadrupling, where it is found that Southern Ocean Ekman transport is essential to reproduce the magnitude and vertical profile of ocean heat uptake.

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Mechanisms of Southern Ocean Heat Uptake and Transport in a Global Eddying Climate Model

Journal of Climate ◽

10.1175/jcli-d-15-0579.1 ◽

2016 ◽

Vol 29 (6) ◽

pp. 2059-2075 ◽

Cited By ~ 23

Author(s):

Adele K. Morrison ◽

Stephen M. Griffies ◽

Michael Winton ◽

Whit G. Anderson ◽

Jorge L. Sarmiento

Keyword(s):

Southern Ocean ◽

Heat Transport ◽

Climate Model ◽

Heat Budget ◽

Global Climate Model ◽

Heat Content ◽

Ocean Heat Uptake ◽

The North ◽

Heat Uptake ◽

Mixed Layers

Abstract The Southern Ocean plays a dominant role in anthropogenic oceanic heat uptake. Strong northward transport of the heat content anomaly limits warming of the sea surface temperature in the uptake region and allows the heat uptake to be sustained. Using an eddy-rich global climate model, the processes controlling the northward transport and convergence of the heat anomaly in the midlatitude Southern Ocean are investigated in an idealized 1% yr−1 increasing CO2 simulation. Heat budget analyses reveal that different processes dominate to the north and south of the main convergence region. The heat transport northward from the uptake region in the south is driven primarily by passive advection of the heat content anomaly by the existing time mean circulation, with a smaller 20% contribution from enhanced upwelling. The heat anomaly converges in the midlatitude deep mixed layers because there is not a corresponding increase in the mean heat transport out of the deep mixed layers northward into the mode waters. To the north of the deep mixed layers, eddy processes drive the warming and account for nearly 80% of the northward heat transport anomaly. The eddy transport mechanism results from a reduction in both the diffusive and advective southward eddy heat transports, driven by decreasing isopycnal slopes and decreasing along-isopycnal temperature gradients on the northern edge of the peak warming.

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The Influence of Sea Ice on Ocean Heat Uptake in Response to Increasing CO2

Journal of Climate ◽

10.1175/jcli3756.1 ◽

2006 ◽

Vol 19 (11) ◽

pp. 2437-2450 ◽

Cited By ~ 94

Author(s):

C. M. Bitz ◽

P. R. Gent ◽

R. A. Woodgate ◽

M. M. Holland ◽

R. Lindsay

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Arctic Ocean ◽

North Atlantic ◽

Deep Ocean ◽

Temperature Gradients ◽

The Arctic ◽

Ocean Heat Uptake ◽

Heat Uptake ◽

The Arctic Ocean

Abstract Two significant changes in ocean heat uptake that occur in the vicinity of sea ice cover in response to increasing CO2 are investigated with Community Climate System Model version 3 (CCSM3): a deep warming below ∼500 m and extending down several kilometers in the Southern Ocean and warming in a ∼200-m layer just below the surface in the Arctic Ocean. Ocean heat uptake caused by sea ice retreat is isolated by running the model with the sea ice albedo reduced artificially alone. This integration has a climate response with strong ocean heat uptake in the Southern Ocean and modest ocean heat uptake in the subsurface Arctic Ocean. The Arctic Ocean warming results from enhanced ocean heat transport from the northern North Atlantic. At the time of CO2 doubling, about 1/3 of the heat transport anomaly results from advection of anomalously warm water and 2/3 results from strengthened inflow. At the same time the overturning circulation is strengthened in the northern North Atlantic and Arctic Oceans. Wind stress changes cannot explain the circulation changes, which instead appear related to strengthened convection along the Siberian shelves. Deep ocean warming in the Southern Ocean is initiated by weakened convection, which is mainly a result of surface freshening through altered sea ice and ocean freshwater transport. Below about 500 m, changes in convection reduce the vertical and meridional temperature gradients in the Southern Ocean, which significantly reduce isopycnal diffusion of heat upward around Antarctica. The geometry of the sea ice cover and its influence on convection have a strong influence on ocean temperature gradients, making sea ice an important player in deep ocean heat uptake in the Southern Ocean.

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